<template>
    <ul class="list">
        <li 
        class="item" v-for="(item) of letters" 
        :key="item" 
        :ref="item" 
        @touchstart.prevent="handleTouchStart"
        @touchmove="handleTouchMove" 
        @touchend="handleTouchEnd" 
        @click="handleLetterClick"
        >
            {{ item }}
        </li>
    </ul>
</template>
  
<script>
export default {
    name: 'CityAlphabet',
    props: {
        cities: Object
    },
    computed: {
      letters () {
        const letters = []
        for (let i in this.cities) {
          letters.push(i)
        }
        return letters
      }
    },
    data () {
      return {
        touchStatus: false,
        startY: 0,
        timer: null
      }
    },
    updated () {
      this.startY = this.$refs['A'][0].offsetTop
    },
    methods: {
        handleLetterClick(e) {
            this.$emit('change', e.target.innerText)
        },
        handleTouchStart() {
            this.touchStatus = true
        },
        handleTouchMove(e) {
            if (this.touchStatus) {
                if (this.timer) {
                    clearTimeout(this.timer)
                }
                this.timer = setTimeout(() => {
                    const touchY = e.touches[0].clientY - 79
                    const index = Math.floor((touchY - this.startY) / 20)
                    if (index >= 0 && index < this.letters.length) {
                        this.$emit('change', this.letters[index])
                    }
                }, 8);
            }
        },
        handleTouchEnd() {
            this.touchStatus = false
        }
    }
}
</script>
  
<style lang="less" scoped>
@import '~@/assets/styles/varibles.less';
.list {
    display: flex;
    flex-direction: column;
    justify-content: center;
    position: absolute;
    top: 1.58rem;
    right: 0;
    bottom: 0;
    width: .4rem;
}
.item {
    line-height: .4rem;
    text-align: center;
    color: @bc;
}
</style>
  